In the U.S. Pat. No. 3,382,460 entitled "Linearly Swept Frequency Generator" issued to Daniel Blitz and Martin Richmond on May 7, 1968, and incorporated herein by reference, a linearly swept frequency generator is disclosed in which the linearity of the generator is carefully controlled. The generator employs a voltage controlled oscillator which produces an output whose frequency sweeps through a given frequency range. The control voltage is varied so as to vary the frequency of the output signal as a predetermined function of time over the sweep frequency range. In order to obtain strict linearity during that sweep, the phase of the output signal is sampled at intervals as determined by a sampling signal. The sampling intervals are chosen so that the phase of the output signal changes by predetermined increments between successive samplings. Generally, the intervals are selected so that the output signal contains only integral numbers of cycles between successive samplings as its frequency varies at a prescribed rate. This enables the sampling of the phase of the signal during its sweep, in terms of the positive going zero crossover of the output signal, which crossovers are to occur at exactly the sampling time, such that by measuring the amplitude difference from zero at the sampling time, the phase error can be detected and corrected. Thus, any variation in the sampled phase from the norm is detected and is fed back as an error signal to the voltage controlled oscillator to correct the slope, phase and frequency of its output signal.
The system described in the above-mentioned patent is particularly suitable for achieving a controlled linear frequency sweep. In this case, the waveform gains or loses cycles at a constant rate. Therefore, a constant sampling frequency can be utilized to check the phase of the continuously varying generator output frequency. For simplicity, the system is arranged to sample the output signal at positive going zero axis crossings. As long as the average value of the signal during the brief sampling interval is zero, no error signal is fed back to the oscillator. However, if a signal has a leading, or lagging phase this produces a finite average voltage which is used as an error signal to correct the output signal.
As such, the above-mentioned linearly swept generator has applications for receivers arranged for surveillance of a frequency band which employs a linearly swept frequency generator as its local oscillator. In these types of receivers, each received signal is timed from the beginning of the frequency sweep and the length of this interval corresponds to the frequency of the signal. Thus, the frequency of incoming signals may be accurately ascertained. In this application and in many others, the accuracy of the method of frequency measurement depends on the accuracy with which the generator is swept.
It has now become increasingly necessary to sweep a given frequency band of interest at higher rates. Moreover, it has been found, that if the sweep rate can be made fast enough, it is possible not only to detect the presence and frequency of an incoming signal, but also to demodulate the signal simultaneously with its detection. Additionally, with increased frequency sweep rates, if it is impossible to demodulate the signal, at least a certain amount of modulation sorting can be obtained without complete demodulation.
However, one cannot merely increase the sweep rate by using faster clocks and sampling rates. This is because the sampling rate is limited by the requirement for integral numbers of cycles between successive samplings. In general, for the system described above, the sample rate can be no greater than the square root of the sweep rate.
In order to solve the sample rate problems and obtain the requisite linearity, it is a part of the subject invention to introduce phase shifts into the portion of the output signal from the voltage controlled oscillator used in the control loop so as to produce additional positive going zero axis crossings. If this is done accurately, the phase of the rapidly swept signal may be sampled more frequently than would be possible utilizing only integral numbers of cycles between samplings. Thus, for instance, if there is in the course of one ordinary cycle only one axis crossing, by the subject technique, it is possible to artificially produce four axis crossings instead of one. This increases the sample rate by a factor of four and permits more rapid sweeping of the voltage controlled oscillator with the requisite linearity. Alternatively, linear accuracy may be increased at a given sweep rate by increasing the number of axis crossings.
It has been found that due to certain symmetries of phase when sweeping through a frequency range, it takes only a small number of discrete phase shifts to accomplish the result of additional zero axis crossings to permit an increased sample rate.
It will be appreciated that once having introduced the appropriate phase shifts, the phase at the increased sample rate may be utilized for the correction of the voltage controlled oscillator in the same manner as described in the above-mentioned patent.
It is, however, only with extreme difficulty that one can directly change the phase of the voltage controlled oscillator output so as to effect additional numbers of zero axis crossings at calculated points in time. It is therefore a portion of this invention to introduce the phase changes which are necessary to effect the zero axis crossings by heterodyning the voltage controlled oscillator output with signals having a number of different waveforms switched in during the sweep. In order to generate signals with these waveforms, the output of a fixed oscillator is phase shifted by discrete amounts during the sweep. This causes the downshifted signal to produce the requisite zero axis crossings and is accomplished in one embodiment by successively delaying the output of the fixed oscillator by fixed amounts during the sweep and by connecting the output of these delay lines to the heterodyning network during the sweep of the voltage controlled oscillator. The switching is controlled by the same sweep timing that controls a ramp generator which, in turn, controls the voltage controlled oscillator. Thus, in synchronism with the sweep, successive discretely delayed fixed oscillator outputs are switched in to be heterodyned with the output of the voltage controlled oscillator. Amplitudes representing variations from zero phase at the calculated zero axis crossing points are measured in the same manner as in the above-mentioned patent and are utilized to effect instantaneous control of the voltage controlled oscillator.
It is a feature of this invention that it is possible to use only a small number of phase shifts or delays. This is because of the discovery of certain symmetries of phase, when sweeping through a given frequency range. A mathematical treatment of how the phase shifts and thus, the delays, are chosen follows in the detailed description of the invention.
It, therefore, is an object of this invention to provide an improved linearly swept frequency generator;
It is another object of this invention to provide a linearly swept frequency generator whose sweep rate may be increased due to the provision of an error detection and feedback circuit which utilizes an increased sample rate, thereby to permit correction of the output of the voltage controlled oscillator even when the oscillator is operated at higher sweep rates;
It is a still further object of this invention to provide a method for increasing the sweep rate of a frequency generator while still maintaining the linearity of the sweep.
These and other objects of the invention will be better understood in connection with the appended drawings taken in connection with the following detaild description wherein.